(1) Field of the Invention
The present invention relates to an optical print head (PH) that performs writing onto a photoreceptor by an optical beam and an image forming apparatus including the optical PH.
(2) Related Art
With a recent increase demand for size-reduction of optical PHs that perform writing onto a photoreceptor by an optical beam that are included in image forming apparatuses such as printers, there have been increasingly used optical PHs in which micro dot light-emitting elements are disposed in a line shape.
For example, Patent Literature 1 (Japanese Patent Application Publication No. 2005-144686) discloses an optical PH that has a configuration in which a plurality of light-emitting elements (organic EL elements), a first power line, and a second power line are provided on a first substrate, and a first auxiliary power line and a second auxiliary power line are provided on a second substrate. The light-emitting elements are arranged in a single line. The first power line is a thin-film wire, and is connected with a feeding point on a power source side. The second power line is a thin-film wire, and is connected with a feeding point on a ground side. The first auxiliary power line is electrically connected with the first power line at a plurality of points. The second auxiliary power line is electrically connected with the second power line at a plurality of points.
According to this configuration with an increased number of feeding points, it is possible to suppress voltage variation of the power line from influencing the light-emitting elements. In other words, it is possible to shorten a length from one feeding point to each of the light-emitting elements compared with the case where a less number of feeding points are provided such as a case where no auxiliary power line is provided. This decreases a potential drop due to a wiring resistance. As a result, it is possible to decrease a difference in drive current to be supplied between the light-emitting elements caused by the potential drop, thereby to decrease a difference in light emission amount between the light-emitting elements. This suppresses image unevenness due to the difference in light emission amount.
However, another problem occurs in the configuration disclosed in Patent Literature 1. Specifically, in order to provide an auxiliary power line, it is necessary to provide a wiring on a sealing plate protecting the light-emitting elements and provide a mechanism for electrically connecting the auxiliary power line with each of power lines. This complicates a wiring configuration and requires a high manufacturing cost.
Also, even if the configuration is adopted in which a certain increased number of power feeding points are provided as in the configuration disclosed in Patent Literature 1, a potential drop still occurs on a part of a power line between each two adjacent power feeding points in a direction in which current flows from the power line. Accordingly, a problem still remains that unevenness in light emission amount due to the potential drop is not sufficiently eliminated.
Also, there has been considered a method according to which, in a single line period in which single line writing is performed onto a photoreceptor by an optical beam, occurrence of a potential drop is prevented by suspending supply of a drive current to all the light-emitting elements to turn off all the light-emitting elements while luminance signals each indicating one of the light-emitting elements are sequentially held in holding elements provided in one-to-one correspondence with the light-emitting elements. However, another problem is caused by the above method. Specifically, the method increases a period in which the light-emitting elements are turned off, and as a result decreases a light emission duty ratio that is a ratio of the light emission period of the light-emitting elements in a main scanning period (Hsync). Therefore, it is necessary to increase a light emission amount of the light-emitting elements in order to a sufficient exposure amount in a short light emission period. This results in a short operating life of the light-emitting elements.
The present invention was made in view of the above problems, and aims to provide an optical PH that is capable of suppressing unevenness in light emission amount between light-emitting elements due to a potential drop on a power line caused by a current flowing through the light-emitting elements from the power line and increasing the light emission duty ratio, and an image forming apparatus that includes the optical PH.